1. Field of the Invention
The present invention relates to multiple channel multiplexers and demultiplexers for fiber optic systems. In particular, the present invention relates to wavelength division multiplexer/demultiplexer systems that provide parallel multiplexing/demultiplexing of a multiple channel optical signal.
2. Background Technology
The increasing demand for bandwidth, coupled with the high cost of laying new optical fiber, has created a strong demand to find new and better ways to increase the carrying capacity on existing optical fiber systems. One such way to increase the capacity is by a technique called wavelength division multiplexing (WDM), which employs multiple wavelengths to carry multiple signal channels and thereby greatly increase the capacity of installed fiber optic networks.
Wavelength division multiplexing (WDM) technology has become a vital component of optical communication systems. In a WDM optical system, light from several lasers, each having a different central wavelength, is combined into a single beam that is introduced into an optical fiber. Each wavelength is associated with an independent data signal through the optical fiber. At the exit end of the optical fiber, a demultiplexer is used to separate the beam by wavelength into the independent signals. In this way, the data transmission capacity of the optical fiber is increased by a factor equal to the number of single wavelength signals combined into a single fiber.
A demultiplexer (DEMUX) device is designed to selectively direct several channels from a single multiple-channel input beam into separate output channels and a multiplexer (MUX) device provides a single multiple-channel output beam comprising the combinations of a plurality of separate input beams. A multiplexer-demultiplexer (MUX/DEMUX) device operates in either the multiplexing or demultiplexing mode depending on its orientation in application, i.e., depending on the choice of direction of the light beam paths through the device.
Thus, in a WDM system, optical signal channels are: (1) generated by light sources; (2) multiplexed to form an optical signal constructed of the individual optical signal channels; (3) transmitted over a single waveguide such as an optical fiber; and (4) demultiplexed such that each channel wavelength is individually routed to a designated receiver such as an optical detector.
Generally, applications for MUX/DEMUX technology include long haul communications and local area data networks. Both digital and analog systems have been demonstrated for voice, data and video. The scope of applications for WDM devices ranges from spacecraft and aircraft applications to closed circuit and cable television systems. In view of these diverse applications, much effort has been expended toward developing WDM technology.
Wavelength selectivity in MUX/DEMUX devices may be achieved through the use of the wavelength-selective characteristics of optical thin film interference filters, such as high and low bandpass filters and dichroic filters. Wavelength selectivity may also be achieved with angularly dispersive devices including prisms and various diffractive grating devices, e.g., prism grating devices, linear grating devices, and chirped grating devices. The grating devices may be of the Littrow-type, which uses a common lens of either a conventional lens type or a graded index (GRIN) rod lens type. No-lens systems are also known and may have, for example, only a concave grating or a combination of a slab waveguide with a grating device. Combinations of grating devices and optical filters are also known.
Conceptually, each wavelength channel in an optical fiber operates at its own data rate. In fact, optical channels can carry signals at different speeds. The use of WDM can push total capacity per fiber to hundreds of gigabytes per second. Generally, more space is required between wavelength channels when operating at 10 gigabytes per second than at 2.5 gigabytes per second, but the total capacities are nonetheless impressive. For example, in the case of 4 wavelength channels at a data rate per channel of 2.5 gigabytes per second, a total data rate of 10 gigabytes per second is provided. With 8 wavelength channels at a data rate per channel of 2.5 gigabytes per second, a total data rate of 20 gigabytes per second is provided. In fact, other wavelength channels can include, for example, 16, 32, 40, or more wavelength channels operating at 2.5 gigabytes per second or 10 gigabytes per second and allow much higher data transfer possibilities. Further, the use of multiple fibers in a single cable can provide even higher transmission rates.
Optical WDM networks typically allocate a portion of the spectrum about a center frequency of the nominal channel wavelength for signal transmission. For example, in dense wavelength division multiplexing (DWDM) systems, channel spacings of less than 1 nm are typically used with wavelength bands centering around 1550 nm. Other systems may require or allow narrower or wider channel widths or spacings. Whereas DWDM is commonly used in telecommunications where the dense channel spacing is ideal, DWDM is normally incompatible with local network data transfer because the narrow channel spacing leads to excessive crosstalk that is unacceptable in data transfer applications.
One solution to crosstalk and channel separation problems in local area networks (LAN), metropolitan area networks (MAN), and wide area networks (WAN) is wide wavelength division multiplexing (WWDM), which is an industry-defined term that indicates narrow bands of wavelengths that are spaced relatively far apart. Typically, the wavelength bands are about 10 nanometers (nm) wide and are spaced about 25 nm apart. The wavelength bands in WWDM bands are centered at about 1310 nm and typically contain four channels at 1275 nm, 1300 nm, 1325 nm, and 1350 nm, each within about xc2x15 nm of the designated wavelength. WWDM can be expanded to up to 100 gigabytes per second or more. Nevertheless, when more than 4 wavelengths, for example 8 or 16, are multiplexed, the demultiplexing needs become greater and the accompanying risk of excessive beam attenuation heightens.
An advantage of the wide channel spacing in WWDM is that it requires no temperature control over the range of 0xc2x0 C. to 70xc2x0 C. This is because, although laser wavelengths may drift by a few nanometers over the range of 0xc2x0 C. to 70xc2x0 C., WWDM has an acceptable wavelength variation of xc2x15 nm. Therefore, WWDM is not particularly limited by temperature conditions.
Similar to WWDM, coarse wavelength division multiplexing (CWDM) is another industry-defined term and is a solution to crosstalk and channel spacing problems. CWDM denotes wavelength bands that are about 10 nm wide and are spaced about 20 nm apart. The CWDM bands are centered at about 850 nm and about 1550 nm.
The 10-gigabit per second Ethernet standard (GbE) is based upon WWDM technology. However, the standard has numerous challenges. Various solutions have been proposed for the 10 GbE standard, including WWDM using multiple wavelength lower speed lasers. Currently, the 10 GbE industry is standardizing on a physical layer transceiver that incorporates WWDM technology. On the transmitter side, the standard uses a multiplexer that combines the output from four lasers at 1,310 nanometers and launches them into an optical fiber. On the receiver side there is a demultiplexer that has an input fiber for the four wavelengths or channels and an optical system with color separation capabilities to divide the four channels. The 10 GbE standard provides physical air solutions to support 65 meters on installed multimode fiber, 300 meters on multimode fiber, 2 kilometers on single mode fiber, 10 kilometers on single mode fiber, and 40 kilometers on single mode fiber. It should be noted that the WWDM physical medium dependent (PMD) solution (10 GBASE-LX4) is the only solution that meets all distance objectives of 10 km or less.
One example of a demultiplexer device is disclosed in U.S. Pat. No. 4,993,796 to Kapany et al. (hereinafter xe2x80x9cKapanyxe2x80x9d), which discloses discrete modules for interfacing optical fibers. Kapany discloses the use of concave gratings and dichroic beam splitters to demultiplex multi-channel beams. However, this approach suffers from several disadvantages. First, a single discrete element is practically limited to two channels. To get more than two channels, these discrete components must be daisy chained together with optical fibers. Second, the method described using dichroic coatings for color separation requires double transmission through the coating for transmitted wavelengths. Third, the method described using dichroic coatings for color separation requires coatings on highly curved spherical surfaces. Fourth, the method described using dichroic coatings for color separation requires the assembly and the precise alignment of several discrete optical elements, many of which are highly curved spherical surfaces.
U.S. Pat. No. 4,441,784 to Korth (hereinafter xe2x80x9cKorthxe2x80x9d) discloses the use of a beam splitter in a paraboloid coupler circuit. Korth discloses the use of two paraboloids that are cut perpendicular to their axis of symmetry. The resulting sectional faces are positioned facing each other with an optical element such as a beam splitter or optical filter inserted therebetween. Optical fibers are inserted at various points on each parabolic surface. Light is emitted from one optical fiber and is reflected to another optical fiber within the same paraboloid or transmitted to a receiving fiber on the opposing paraboloid. This approach is limited to couplers using optical fibers to relay the optical signals.
One way to increase the data transfer capability of optical fibers is to add additional optical channels. The current understanding of how to increase the number of optical channels in a multiplexer/demultiplexer device consists of adding additional multiplexer/demultiplexer elements in series. Unfortunately, this approach is limited by several drawbacks. As light beams travel within a demultiplexer, they are attenuated by a variety of mechanisms. For example, in a polymeric-based demultiplexer the beam is attenuated by the polymeric material as it travels within the demultiplexer. As more optical channels are demultiplexed in series, the beam must travel a greater distance through the polymeric material, thus increasing the beam attenuation. Also, each reflection in an optical path, whether it is from a reflective surface or a filter surface is less than one hundred percent because Fresnel reflection losses at each surface interface reduce the overall beam intensity. If the overall beam attenuation becomes too high, the optical channels can no longer be reliably demultiplexed and the number of channels to be separated must be limited.
Accordingly, there is a need for improved multiplexing and demultiplexing devices and methods that overcome the above drawbacks.
It is an object of the present invention to provide a compact, cost effective demultiplexing system that can meet the wavelength demultiplexing requirements for a 10 Gb/sec or faster optical transceiver.
It is a further object of the present invention is to provide a demultiplexer device capable of separating multiple wavelength channels with minimal attenuation.
Another object of the invention is to provide a demultiplexer device capable of separating numerous multiple wavelength channels.
To achieve the forgoing objects and in accordance with the invention as embodied and broadly described herein, multiple channel optical multiplexing/demultiplexing systems are provided which have the property of allowing multiplexing/demultiplexing of a single multi-wavelength beam to occur in parallel by dividing an input multiple wavelength beam into two separate multi-wavelength beams.
In one embodiment, a multiple channel optical demultiplexing system utilizes a beam splitter to divide a single multiple wavelength beam of optical energy into two multi-wavelength beams. A pair of demultiplexers are configured to receive and separate the two multi-wavelength beams into a plurality of wavelength channels. The two multi wavelength beams are thus demultiplexed in parallel, allowing greater efficiency by avoiding excess beam attenuation. The individual demultiplexers can be incorporated into a single unitary device or can be optically interconnected as separate parts.
In another embodiment, the demultiplexing system can use multiple beam splitters to divide a single input multiple wavelength beam into multi-wavelength beams for parallel demultiplexing in a plurality of demultiplexers.
The embodiments of the present invention can also be used as multiplexing systems by reversing the direction of the multiple channel beams. Thus, a multiplexing system according to the invention includes a first multiplexer configured to receive and combine a first plurality of wavelength channels into a first multi-wavelength beam, and a second multiplexer configured to receive and combine a second plurality of wavelength channels into a second multi-wavelength beam. A beam splitter is in optical communication with the first and second multiplexers and is adapted to combine the first multi-wavelength beam with the second multi-wavelength beam to produce an output multiple wavelength beam.
The foregoing objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.